The function means of an echo canceller is formed mainly of a transversal filter and a coefficient modifier part that successively updates the coefficient of the filter. Generally, the normalized least mean squares (NLMS) algorithm, which is excellent in stability and convergence, relatively simple in terms of operations, and actually realized as a device using an LSI, may be employed as a method of estimating, or updating, the filter coefficient {hi} of the transversal filter.
Further, in order to realize full duplex hands-free communication, the echo canceller requires, as its important functions, an echo canceller protection function and a nonlinear processing (center clipper) function. The echo canceller protection function prevents errors from being caused by the speech of a near-end talker in updating the tap coefficient (filter coefficient) at the time of two-way simultaneous communication (double talk). The nonlinear processing function unconditionally suppresses a low echo signal to a zero value in order to increase the echo suppression effect when the echo level is low.
It is difficult to cancel echoes completely in a conventional common echo canceller that updates the filter coefficient (tap coefficient) of the transversal filter using the NLMS algorithm. This is because the conventional echo canceller treats a speech signal as its main target while assuming an uncorrelated signal, and cannot always secure a sufficient tap length of the transversal filter with respect to the reverberation time of a room. Accordingly, the above-described nonlinear processing is often employed.
The nonlinear processing aims to reduce residual echo by adding a so-called voice switch that turns ON or OFF depending on a signal level. Japanese Laid-Open Patent Application No. 4-150127 discloses such a technique.
Further, Japanese Laid-Open Patent Application No. 10-285083 discloses a device that includes means for varying a clip level and changing transmitting attenuation depending on the condition of transmission.
These voice switching methods can reduce the residual echo, but have difficulty in dealing with the condition of double talk, so that there occurs the problem characteristic of the voice switch that the leading part of the speech of a near-end talker is cut off. In order to solve the above-described problem and ensure further reduction in the residual echo, a technology disclosed in Japanese Laid-Open Patent Application No. 9-162787 has been proposed.
Japanese Laid-Open Patent Application No. 9-162787 discloses a configuration that employs a low-pass filter setting a low cut-off frequency when signal power is low and setting a high cut-off frequency when signal power is high. The residual echo of an echo-canceled signal includes a high-frequency component. Offensive noise can be reduced by suppressing the high-frequency component. The low-pass filter is employed as means for suppressing the high-frequency noise component.
Further, noise suppression means includes power calculation means for calculating the power level of an input signal supplied from an echo cancellation part, comparison means for comparing the power level of the signal calculated by the power calculation means with a predetermined threshold for noise determination, and noise suppression means for performing noise suppression on the signal by the low-pass filter when the comparison result shows that the power level of the signal is lower than or equal to the predetermined threshold.
The low-pass filter of the noise suppression means varies the cut-off frequency so that the cut-off frequency is low when the signal power is low as residual echo and the cut-off frequency is high when the signal power is high as the speech signal of a near-end talker. Specifically, the low-pass filter of the noise suppression means is realized by moving average processing, and the variation in the cut-off frequency of the low-pass filter is realized by a variation in the moving average interval length of the moving average processing. Thereby, the low-pass filter (LPF) with a variable cut-off frequency of a simple configuration can be realized.
The power calculation means obtains the power of the signal expressed in the form of the exponent of 2n by digital processing. The comparison means obtains the number of moving average interval length bits m by subtracting the power level in the form of the exponent of 2n from the noise determination threshold in the form of the exponent of 2n. The noise suppression means performs, by digital processing, moving average processing with respect to an interval length determined by the obtained number of moving average interval length bits m when m is not negative.
This technology, however, contains the following problems. That is, the noise suppression means compares the output power of the echo cancellation part with the predetermined determination threshold. The determination threshold can be determined with no problem when the amount of residual echo or a near-end input power value can be estimated in advance. In the case of a great environmental variation, however, it is difficult to distinguish between the residual echo and the speech of the near-end talker, thus making it difficult to determine the determination threshold.
In the case of a fixed threshold for noise determination, if the noise determination threshold is fixed to a high value, any signal that is smaller than the set threshold goes through the noise (residual echo) suppression means that is the LPF with a variable cut-off frequency. At this time, the lower the input near-end power, the lower the cut-off frequency of the LPF. Therefore, if the threshold setting lacks deliberation, low-power fricative consonants may be cut by bandwidth restriction.
On the other hand, if the determination threshold is fixed to a low value, the function of suppressing residual echo does not work. This is because the residual echo temporarily increases at the early learning stage of the echo canceller or at the time of echo path change. When far-end input speech increases, it is natural that echo should increase in amount. However, signal distortion may occur depending on the characteristics of a loudspeaker on the near-end side, so that the echo path characteristic becomes nonlinear, thus increasing the residual echo. The residual echo power exceeds the threshold in such a case, thus resulting in the problem that the residual echo is transmitted to the far-end side without going through the noise suppression part.
In order to cope with this problem, the determination threshold may be set to a high value so that such residual echo goes through the noise suppression part. This, however, causes another problem of the loss of the low-power signal of the near-end talker. Thus, the residual echo and the speech of the near-end talker cannot be distinguished from each other by simple power comparison, so that the noise suppression means does not function properly.
This problem is caused in a system where an echo coming out from a speaker to be input to a microphone has approximately the same magnitude as that of the speech of the near-end talker. For instance, this problem is caused in a handy phone system that requires a loudspeaker and a microphone to be arranged relatively close to each other or in an on-vehicle hands-free telephone system that outputs sound at high volume from a loudspeaker and requires a microphone to be arranged at a distance from the mouth of a speaker.
With respect to the noise suppression means using the LPF with a variable cut-off frequency, Japanese Laid-Open Patent Application No. 9-275367 discloses another technology.
Japanese Laid-Open Patent Application No. 9-275367 discloses an echo canceller device has the functions of (a) detecting the direction of communication depending on the condition of communication from an input signal from the far end (the speech of a far-end talker) and an input signal from the near end (the combination of the speech of a near-end talker and the echo sound of the speech of the far-end talker) and (b) gradually changing the cut-off frequency of an LPF for residual echo suppression every time the direction of communication changes.
Actually, however, it is difficult to detect the direction of communication under a noisy environment or in the case where the direction of communication changes frequently, so that the residual echo is transmitted to the far end if the cut-off frequency is delayed in changing with respect to a change in the direction of communication.